THE NATURE OF ENZYMES 117 



THE CHEMICAL AND PHYSICAL NATURE OF ENZYMES 



Little is known regarding the chemical nature of enzymes, 

 because all attempts to isolate them in a state of purity have 

 hitherto failed. In fact, there is nothing to give certainty that 

 at the end of any process the product in the case of such com- 

 plicated substances is pure, a remark which applies equally to 

 ordinary proteids. None of the criteria of purity in the case of 

 an ordinary crystalloid apply in the case of a colloid of complex 

 constitution, except constancy of percentage composition. It does 

 not crystallise out, 1 it has no definite melting-point, it does not 

 affect the freezing-point or boiling-point, 2 it cannot be synthesised 

 by reactions which can be followed in their course, and it is 

 probably polymerised to a high degree. Hence it would be more 

 correct to say that we do not know whether proteids and enzymes 

 have ever been prepared in a state of purity, rather, than is 

 usually done, that they never have been so prepared. 



One of the great difficulties in freeing enzymes from other 

 substances, such as proteids, is that they share the common 

 property of colloids, of being easily thrown mechanically out of 

 solution, by electrolytes or organic precipitants of proteids. 



The methods of attempted separation have differed in the case 

 of different enzymes and cannot all be gone into in detail here. 



One general method is that of allowing the ferment to digest 

 out any substratum present naturally with it as much as possible 

 where it can be made to do this, as in the case of the proteo- 

 clastic enzymes, and then to precipitate it by means of some in- 

 different precipitate such as calcium phosphate in the case of 

 pepsin, or by the addition of collodion or cholesterin dissolved 

 in a mixture of alcohol and ether. Another method when the 

 ferment does not rapidly undergo alcohol coagulation is to remove 

 the accompanying proteid by allowing the mixture to stand for 

 some weeks under alcohol, and then dissolve the ferment by means 

 of water, as in the case of fibrin ferment (thrombase). Another 

 method is to allow the strong solution to remain standing at the 

 freezing-point for some days, when the enzyme falls out in granular 



1 Unless when in combination with other bodies which confer crystalloid 

 properties. 



2 At least to such an extent that measurements can be made. 



